In LTE (Long Term Evolution) system and the previous wireless communication system, there is one carrier in a cell. The maximum bandwidth in LTE system is 20 MHz, as shown in FIG. 1. For LTE system is a signal-carrier system, when making handover decision, source eNB only provides target eNB with the information of one cell for target-side eNB to make admitting decision.
In LTE-A (LTE Advanced) system, the peak rate is greatly improved compared with LTE, which is required to be 1 Gbp at downlink and 500 Mbps at uplink. Only using the carrier with maximum bandwidth of 20 MHz can hardly achieve the requirements of peak rate. Therefore, LTE-A system needs to expand the bandwidth available to terminal, thus the introduction of CA (Carrier Aggregation) technology, that is, aggregate several continuous or discontinuous carriers under the same eNB (evolved Node-B) and serve UE (User Equipment), so as to provide the required rate; thus these aggregated carriers are also called CC (Component Carrier). Each cell can be a cell, and those cells (CCs) under different eNB cannot be aggregated. To ensure UE of LTE can be work under each aggregated carrier, each carrier cannot exceed 20 MHz at most. CA technology of LTE-A is shown in FIG. 2, where there are 4 carriers that can be aggregated under the eNB of LTE-A shown, and the eNB can perform data transmission on 4 carriers and UE simultaneously, to improve system throughput.
Handover of LTE system can be divided into two categories from the perspective of the quantity of participating nodes, that is, intra-eNB handover and inter-eNB handover. Therein, intra-eNB handover can be the handover performed for updating key parameters for security reasons. As to inter-eNB handover, in view of the type of interface between source eNB and target eNB, it can be classified into X2 handover and S1 handover, to transmit message respectively through X2 interface and S1 interface, so message of S1 handover will pass through MME node. And for UE of LTE system works under single carrier, the handover is that between signal-carrier cells.
In LTE-A system, UE can simultaneously aggregate to perform signaling scheduling and service transmission in multiple carriers. When UE uses CA, each aggregated component carrier refers to a cell. To differentiate such cells, the concept of primary cell (Pcell) and secondary cell (Scell) are introduced. Pcell has the following characteristics: PUCCH (Physical Uplink Control Channel) only exists on Pcell; in case of radio link failure of Pcell, UE triggers RRC (Radio Resource Control) to connect re-establishment; system information acquisition and updating of Pcell are the same as LTE process; NAS information is acquired through Pcell; random access is performed only on Pcell. UE has only one Pcell but one or more Scell (s). The frequency/carrier of Pcell are called PCC (Primary Component Carrier) and correspondingly that of Scell is called SCC (Secondary Component Carrier). For carrier aggregation characteristics are introduced into LTE-A system, UE can simultaneously aggregate to perform signaling scheduling and service transmission in multiple carriers, which requires source eNB to provide more information to target eNB in the process of multi-carrier handover.
In current technology, UE only can provide measurement result of one or several cells at one frequency in each measurement report. As to UE in CA state, if source eNB intends to trigger multi-carrier handover, it needs to wait for measurement report results many a time, which will increase the total measurement report delay, cause deferring of handover decision or UE losing connection and further induce call drop; if source eNB directly decides handover without waiting for repeated measurement result report, it will be unable to provide appropriate candidate cell list to target eNB, thus making target eNB blindly select target cell when having no idea of channel quality and further increasing the probability of handover failure.